With a wide spreading use of information terminals, there is a growing need for a flat panel display for use in a computer. In this regard, a display medium of the flat panel display device is generally provided by using an element such as a liquid crystal, an organic EL (organic electroluminescence) and an electrophoresis. In such display medium, a technology using an active drive element (for example, a field-effect transistor like a TFT element) as an image drive element has been becoming a primary means to secure a uniformity of the screen luminosity, a screen rewriting speed and so forth. The term “TFT” is an abbreviation of “Thin Film Transistor”. For example, the general display device for use in the computer is configured to have the TFT elements on a substrate where a liquid crystal element, an organic EL element or the like is sealed.
Although semiconductors such as a-Si (amorphous silicon) are mainly used in the TFT element, the a-Si can not exhibit a high mobility (for example, the mobility of the a-Si is no more than about 0.5). Accordingly, it is becoming difficult for such semiconductors to comply with the trend toward greater sizes and an improved high-speed driving of the display.
Under these circumstances, an oxide semiconductor is becoming an attractive semiconductor since it exhibits a high performance and can be produced without difficulty. In general, a vacuum processes (e.g., sputtering process and ALD process) and a solution processes are used to produce a metal oxide film. In this regard, a production of the oxide semiconductor has a tendency toward making use of the sputtering process.